This invention relates generally to radiotelephones and, in particular, to radiotelephones, mobile stations and wireless communicators, including those capable of operation with a cellular network, that employ a closed loop transmitter power control system.
The control of the RF transmitter power in a mobile station is an important consideration, as mobile stations are expected to operate at or very close to prescribed RF transmitter power levels. Furthermore, during a call the mobile station may be commanded by a base station to adjust the RF transmitter power. For example, if a mobile station in a vehicle begins a call with a relatively high power level at the edge of a particular cell, and then approaches the center of the cell, the mobile station will likely be required to decrease its RF transmitter power. Failure to properly control the transmitter power can result in, for example, an occurrence of dropped calls and/or the generation of interference that adversely affects other users. Transmitter power control is important in all types of wireless communication systems, including FM (analog) systems, as well as digital TDMA and CDMA systems.
Another important aspect of RF power control is an ability to completely turn off the RF transmitter when commanded or desired, thereby outputting, ideally, zero transmitted RF power.
FIG. 3 shows a simplified diagram of a conventional closed loop transmitter power control system of a type that is typically found in mobile stations. The closed loop transmitter power control system feeds an antenna through a directional coupled from an RF power amplifier. The RF power amplifier amplifies an input RF signal, which is modulated in some way with appropriate signalling information and, during a voice call, the user""s voice. Data calls can be made in the same way (e.g., when a computer modem connects to a data communications network, such as the Internet). The purpose of the coupler 32 is to extract a small part of the transmitted RF energy and provide this energy to a detector, which functions to convert the RF output of the coupler to a DC signal having a magnitude that is a function of the output RF power level. The detector output signal is applied to one input of a closed loop error amplifier, which receives a power control voltage at another input. The power control voltage, which may be set by a controller (a microprocessor) using a digital to analog converter (DAC), is representative of a desired output RF power level. The output of the error amplifier is a difference between (i.e., the error) in the commanded RF output power level and the actual output RF power level, as reflected by the DC signal that is output from the detector. The error signal from the closed loop error amplifier is then used to control the gain of the RF amplifier, thereby closing the power control loop. When the output RF power level at the input to the antenna, as indicated at the output of the detector, equals the commanded RF power level, as indicated by the power control voltage output from the DAC, the closed loop has obtained equilibrium by setting the output of the error amplifier to a proper potential.
General reference in this regard can be made to the following commonly assigned U.S. Patents: U.S. Pat. No. 5,230,091 issued Jul. 20, 1993, entitled xe2x80x9cMethod And Apparatus For Tuning And Compensating Power Levels In A Radio Telephonexe2x80x9d, by Risto Vaisanen; U.S. Pat. No. 5,276,917 issued Jan. 4, 1994, entitled xe2x80x9cTransmitter Switch-On In A Dual-Mode Mobile Phonexe2x80x9d by Petteri Vanhanen et al.; U.S. Pat. No. 5,548,616, issued Aug. 20, 1996, entitled xe2x80x9cSpread Spectrum Radiotelephone Having Adaptive Transmitter Gain Controlxe2x80x9d by Lars H. Mucke et al.; and U.S. Pat. No. 5,697,074 issued Dec. 9, 1997, entitled xe2x80x9cDual Rate Power Control Loop For A Transmitterxe2x80x9d by Eero Makikallio et al.
Based on the foregoing discussion it can be appreciated that the overall calibration or alignment of the RF power control loop is an important consideration, particularly when making factory and field office calibrations. Typically expensive and complex test equipment, such as RF power meters and the like which also require periodic calibration, are required to actually measure the RF power output at various levels. The measured RF power is then compared to the expected (commanded) power, and calibration constants are derived and stored in a memory of the mobile station for later use. The calibration constants are used to null out any offsets in the RF power control loop circuitry, such as those exhibited by the DAC. In that the gain of the error amplifier is typically in the range of about 10 to 100,000, it can be appreciated that even a small offset voltage in the DAC""s output can result in a large shift in the output of the closed loop error amplifier, thereby causing the actual RF transmitter power to deviate from the desired value.
Attention to other drift sources, namely the error amplifier and the power detector, can be considered as well. Additionally, the calibration constants are used to provide the desired output power levels. Typically, one of the constants indicates a xe2x80x9cknee valuexe2x80x9d level that just sets the loop at equilibrium, but does not increase the RF output power. The other constants indicate the various desired output power levels. For a power ramp the output of the DAC is ideally ramped from the knee value level to the desired output power value, and then back to the knee value level for ramp down. The knee value, ideally, compensates the various loop offsets, while the other constants are usually defined with reference to the knee value. The knee value may thus also be referred to as a xe2x80x9cbase valuexe2x80x9d.
As such, it can be appreciated that finding the knee value simplifies the task of determining the power level dedicated constants, as the latter can be simply added to the knee value. The power level dedicated constants then depend on the conversion coefficient of the DAC, the coupling factor and the impedance of the directional coupler, and the signal envelope sensitivity of the power detector. If these parameters are sufficiently constant, the calibration of the power level dedicated constants may not be required (depending, of course, on the accuracy requirement for the output power). In this case a calibration of the knee value may suffice, and predetermined values for the xe2x80x9cadd onxe2x80x9d power level constants then applied. It can thus be appreciated that the knee value is a very sensitive parameter, and important in the sense that all power levels use it as a starting point for a power ramp.
With DACs incorporating some kind of level shifting circuitry, the output voltage drift with reference to a given input code may be considered as an offset drift of the DAC. More generally, however, the DAC output may be considered as simply drifting (not an offset drift per se) with temperature and aging. From the power control loop point of view this drift appears as an offset drift around the control voltage knee value, i.e., the value needed to set the loop just at equilibrium without causing an increase in the RF output power.
A small offset (or drift) in any of the relevant loop parameters near to the power control voltage knee value (such as at the start of a power ramp up or when using a small output power level) is detrimental. This is due to the typically very small detector voltage obtained under these conditions (for zero RF output power the envelope voltage component of the detector is zero, and increases with increasing RF power). In general, the directional coupler is a linear device and does not incorporate an offset or nonlinearity error.
In order to be able to shut the RF power down completely, a xe2x80x9czero codexe2x80x9d output of the DAC must be below a level defined by the detector bias voltage and the input offset of the loop error amplifier. The offset alignment procedure is run in order to determine a value for a digital control word that just balances the RF control loop, while not generating any significant RF output at the antenna.
The offset alignment is often done by adjusting the control word until a given low RF output level is obtained. The offset word is then estimated by deducting a predetermined amount of quantization steps from the word, yielding the desired RF power.
A partial solution to the offset drift problem can be obtained by specifying a low drift DAC, error amplifier, and power detector. An alignment of the power control voltage knee value will be needed, unless components with extremely good absolute accuracy can be used. However, this requires the use of more expensive components, which is not always desirable.
It is thus a first object and advantage of this invention to provide an improved method for calibrating or aligning the RF power control loop of a wireless communicator.
It is another object and advantage of this invention to provide an improved method for calibrating or aligning the RF power control loop of a wireless communicator, and which does not require that an RF power measurement be made.
It is a further object and advantage of this invention to provide an improved method for calibrating or aligning the RF power control loop of a wireless communicator, and that eliminates or reduces a need to provide external test equipment, such as RF power meters.
It is one additional object and advantage of this invention to provide novel embodiments of an RF power control loop that is capable of operating with a resident microprocessor for accurately determining the zero code for a DAC which generates a power control voltage, both for initial alignment purposes as well as for real time, long term drift compensation purposes.
It is another object and advantage of this invention to provide an improved method for calibrating or aligning the RF power control loop knee value.
The foregoing and other problems are overcome and the objects and advantages of the invention are realized by methods and apparatus in accordance with embodiments of this invention.
This invention provides a method and apparatus for aligning the knee voltage value of the RF power control loop. For this calibration no RF measurements or RF power meters are required. Finding the knee value has typically been a complex procedure and the most time consuming part of the RF power control loop calibration procedure. The teachings of this invention therefore aid the entire calibration process, as the power level dedicated constants are more readily calibrated after the knee voltage has been found. For a well designed power control loop the need to calibrate the power level constants depends on the accuracy of the power detector envelope sensitivity. Thus, the teachings of this invention may not alone eliminate the use of RF power meters for calibrating the power level constants, but makes this possible provided the loop components are sufficiently accurate.
A method is disclosed for determining a calibration value for a RF power control loop. The RF power control loop includes an error amplifier having an output for outputting an indication of a difference between an actual RF output power and a desired RF output power. The method includes steps of (a) commanding an RF output power; (b) measuring the output of the error amplifier; (c) comparing the measured output of the error amplifier to a reference value; and (d) iterating the steps of commanding, measuring and comparing until some predetermined number of iteration cycles has been completed.
A desired RF output power level may be zero, and the reference value has a magnitude that is a function of an RF power amplifier gain control voltage. More particularly, the reference value has a magnitude that is less than a magnitude of the RF amplifier control voltage threshold level (or knee level), above which the RF amplifier begins to output RF energy. In a preferred embodiment of this invention the reference value has a magnitude that is about one half of a magnitude of the RF amplifier control voltage threshold level above which the RF amplifier begins to output RF energy.
The step of commanding includes a step of applying a digital control word to an input of a digital to analog converter (DAC). The DAC has an output coupled to an input of the error amplifier, and another input of the error amplifier is coupled to the output of an output RF power detector.
In one embodiment of this invention the steps of measuring and comparing are performed by monitoring an output of a comparator circuit that has a first input coupled to the output of the error amplifier and a second input coupled to a voltage potential that is set at the reference value.
In another embodiment the step of measuring is performed by monitoring an output of an analog to digital converter (ADC) circuit that has an input coupled to the output of the error amplifier.
The steps of commanding, measuring, comparing and iterating can be performed during an initial manufacturing (factory) calibration of a wireless communicator, or they can be performed during an initial power-on sequence of the wireless communicator. In addition, the steps of commanding, measuring, and comparing can be performed between transmitted bursts of RF power, such as occur in TDMA systems.
In accordance with a presently preferred search algorithm, the step of iterating operates by continuously dividing a search range until the remaining search range is one DAC input control word quantization step, or some integer number of DAC quantization steps greater than one (for a lower accuracy requirement application).